Overlapping data instances can segfault

When exploring data families for some fast unboxed tuple representation, we came across the following scenario.

class Foo a where
    data Bar a :: *

instance Foo a where
    data Bar a = PBar a

instance Foo () where
    data Bar () = PUnit

GHCi, version 6.10.1: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( Foo.hs, interpreted )
Ok, modules loaded: Main.
*Main> case PUnit of PBar x -> x
Segmentation fault

It appears the compiler is allowing the refinement of a into () because they overlap, when it shouldn't.

This is less academic than this example, because it affects doing similar operations to automatically unbox tuples of primitives when possible.

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
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Blocking
CC	ekmett@gmail.com
Operating system
Architecture